I used to have VS-100 for 6 years and never changed the ECC83 but although I played a lot I never turn the volume past halfway and only a couple of times I crank it halfway and that was moslty at gigs...Now I own a 79' Fender Twin Reverb...the tubes are original fender labeled 6L6GC's,12AT7s,7015s and a 12AX7A but I'm they're replaced a few times cause it sounds brilliant...this is my first true tube amp and I wonder when to change the power tubes...I think when I sense some corruption in sound and when the volume level reduces severely...well any advice would be appreciated